STAT (Signal Transducer and Activator of Transcription) is a molecule that is activated by stimulation with cytokine(s). STAT-induced inhibitor of STAT function-1 is a protein that is induced by STAT and inhibits cytokine signaling through suppressing activation of Janus Kinase (JAK). STAT-induced inhibitor of STAT function-1 (hereinafter, simply referred to as “SSI-1”, though the abbreviation “SSI-1”, “SOCS-1” or “SIIS-1” have been used) was first isolated from mouse by the inventors of the present invention (PCT Publication No. WO98/30688 and Naka, T. et al, Nature, 387, 924-929 (1997)). So far, eight members of the SSI family molecule, including SSI-2 and SSI-3 (Minamoto, S. et al., Biochem. Biophys. Res. Commun., 237, 79-83 (1997)),have been identified as an inhibitor. The inventors of the present invention have also already isolated human SSI-1 (Minamoto, S. et al., Biochem. Biophys. Res. Commun., 237, 79-83 (1997)).
The functional domain of SSI-1 was determined by an experiment using mutant genes of SSI-1 (Narazaki, M. et al., Proc. Natl. Acad. Sci. U.S.A., 95, 13130-13134 (1998)). More specifically, it is known that 24 amino acid residues existing in N-terminus of SH2 domain (pre-SH2) as well as SH2 domain are essential for the suppression of phosphorylation of JAK, and that SSI-1 associates to JAK through its SH2 and pre-SH2 domains. Moreover, part of the biological function of SSI-1 in vivo has been identified by the inventors of the present invention using SSI-1-deficient mice. In SSI-1-deficient mice, apoptosis was detected in several organs such as lymphocytes, heart and liver, in which SSI-1 was expressed as the ages passed, indicating that SSI-1 closely relates to survival of a cell, especially to an inflammation (Naka, T. et al., Proc. Natl. Acad. Sci. U.S.A., 95, 15575-15582 (1998)).
There are many signaling pathways in a cell, including ones induced by hormones and the like as well as cytokines. For example, insulin is a hormone relating to cell proliferation, the uptake and metabolism of glucose, the synthesis and storage of neutral fat, and protein synthesis. Insulin mediates its biological effects through its specific receptor, insulin receptor (IR). It is known that IR has an intrinsic tyrosine kinase activity and participates in the phosphorylation of insulin receptor substrate 1 (IRS-1) (Paz, K. et al., J. Biol. Chem., 272,29911-29918(1997)). Phosphorylated IRS-1 mediates various biological activities (White, M. F., Diabetologia, 40(Suppl.), S2-S17 (1997) and Myers, M. G. et al., Trens Biochem. Sci. 19 (7), 289-293 (1994)). It is also reported that JAK as well as IR was activated by insulin stimulation, suggesting an involvement of JAK in insulin signaling pathway (Giorgetti-Peraldi, S. et al., Eur. J. Biochem., 234 (2), 656-660 (1995) and Gual, P. et al., Endocrinology, 139 (3), 884-893 (1998)).
However, insulin signaling pathway has not yet been specified, and all molecules involved in the pathway have not yet been identified. It is considered that most of insulin resistant diabetes (which is characterized by normal insulin secretion but hyperinsulinemea and hyperglycemia due to low efficiency of blood sugar incorporation in peripheral tissues because of the low reactivity to insulin) is caused by the functional deficiency of IR. The cause of the disease, however, have not yet been specified and there are no established therapeutics or agents so far.
As mentioned above, the identification of molecules involved in insulin signaling pathway and agents for insulin resistant diabetes through regulating the function of those molecules has been desired. A subject of the present invention is to provide a method for screening a SSI-1 inhibitor that is a molecule capable of inhibiting intracellular insulin signaling.